If the Microsoft SQL Express databases was used it will be migrated to the PostgreSQL database during the upgrade.
The bundled PostgreSQL database can be used for environments of up to 20 hosts and 200 virtual machines.

The user account the vCenter Server service is running as requires the following permissions:

Member of the Administrators group

Log on as a service

Act as part of the operating system

vCenter Server Appliance Requirements in the vSphere Upgrade Guide on page 33.
vCenter Server Appliance 5.1U3 and vCenter Server Appliance 5.5 can be upgraded to vCenter Server Appliance 6.
VMware vCenter Server Appliance can be deployed only on hosts that are running ESXi version 5.0 or later.
If an external vCenter SSO is used, the vCenter Server Appliance cannot be upgraded. Upgrade is only supported if the embedded vCenter SSO is used.
The vCenter Server Appliance PostgreSQL database supports up to 1000 hosts and 10,000 virtual machines.
An Oracle 11g database or an Oracle 12c database are the only external databases supported by the vCenter Server Appliance.

vCenter Server 6 can manage ESXi 5.x hosts in the same cluster with ESXi 6.0 hosts, but not with ESX 4.x of ESXi 4.x hosts.
vCenter Server 4.x cannot be directly upgraded to vCenter Server 6. vCenter Server 4.x must first be upgraded to vCenter 5.x.

Deploying the vCenter Server Appliance requires the Client Integration Plug-In. This is an HTML installer for Windows which can be used to connect directly to an ESXi 5.x or ESXi 6.x host to deploy the vCenter Server Appliance on the host.
vCenter Server Appliance Upgrade Tasks:

Download the .iso installed for the vCenter Server Appliance and Client Integration Plug-in

Install the Client Integration Plug-in from the vcsa directory on the .iso

Upgrade the vCenter Server Appliance using vcsa-setup.html on the .iso

During the vCenter Server Appliance upgrade the following tasks are performed:

Export of the existing vCenter Server Appliance Configuration

Deployment of the vCenter Server Appliance 6.0

Migration of services and configuration data to the new vCenter Server Appliance 6.0

The legacy vCenter Server Appliance is powered off

A few really great blog posts on the vCenter Server Appliance upgrade process:

– Identify the methods of upgrading vCenter
Embedded Deployment Model – The Platform Service Controller and the vCenter Server are installed on the same machine.
External Deployment Model – The Platform Service Controller is installed on a separate machine from the vCenter Server.

vCenter 5.5 and earlier deployed using Simple Install option will be upgraded to vCenter Server with embedded Platform Services Controller.
If vCenter Single Sign-On was on a different machine than vCenter Server, the upgrade will be an external deployment model.
If vCenter Single Sign-On was on the same node as vCenter Server, the upgrade will product an embedded deployment model.

Upgrade including an AutoDeploy Server
If configuration includes an Auto Deploy server, the upgrade process upgrades it when upgrading the associated vCenter Server instance. Auto Deploy server included with an earlier version of the product cannot be used in conjunction with vCenter Server 6.0. If the Auto Deploy server is running on a remote system, it is upgraded and migrated to the same system as vCenter Server during the upgrade process. Settings are migrated to the new location. ESXi hosts must be reconfigured to point to the new Auto Deploy location.

Upgrading with Remote Web Client Server
If configuration includes a remotely deployed vSphere Web Client, it is upgraded along with the vCenter Server instance to which it is registered and migrated to the same location as the vCenter Server instance.

To Collect vCenter Server Appliance Logs from the bash shell run vc-support.sh script to generate the log bundle. A .tgz file will be generated in /var/tmp

To determine which firstboot script failed cat the /var/log/firstbootStatus.json file. This will assist in identifying the firstboot script which failed. Log files for the firstboot scripts are also located in /var/log.

vHersey

Hersey Cartwright is an IT professional with extensive experience designing, implementing, managing, and supporting technologies that improve business processes. Hersey is Solutions Architect for HPE SimpliVity covering Virginia, Washington DC, and Maryland. He holds the VMware Certified Design Expert (VCDX-DV #128) certification. Hersey actively participates in the VMware community and was awarded the VMware vExpert title in 2016, 2015, 2014, 2013, and 2012. He enjoys working with, teaching, and writing about virtualization and other data center technologies. Follow Hersey on Twitter @herseyc